1. Field of the Invention
This invention relates to an overvoltage arrester for protecting electric machines and transformers wherein the arrester comprises an insulating housing and a series circuit disposed in the housing and including a spark gap and a voltage dependent resistor.
2. Description of the Prior Art
Typically, in overvoltage arresters of the above type, the insulating housing comprises a hollow insulator, for example, a porcelain tube, provided with exterior ribs (see, for example, German Auslegeschrift No. 20 56 526). Such an arrester configuration has also been used in cases wherein the voltage dependent resistor comprises zinc oxide (see, for example, German Offenlegungsschrift No. 23 34 420).
On the other hand, overvoltage arresters, particularly for use in networks with low operating voltages, are also known in which the effective elements thereof i.e., the voltage-dependent resistor and the spark gap, are embedded in a hardened cast-plastic compound which simultaneously serves as the arrester housing. In manufacturing this type of overvoltage arrester, a mold is required so that the cast body can be set in proper shape.
It is an object of the present invention to provide an improved overvoltage arrester.